Sean Penn has been lined up to play gay 1970s San Francisco politician Harvey Milk, and Matt Damon his assassin, in a new film from director Gus Van Sant.
Penn is due to star alongside Matt Damon in the new film
Van Sant hopes to begin filming once a distribution deal is finalised.
But the uncertain start date may affect Damon's participation as Milk's killer, Dan White, trade paper Hollywood Reporter said.
Milk, who was assassinated in 1978, was the first openly gay city supervisor of San Francisco.
Van Sant has been lined up to direct a biopic of Milk for over 10 years.
Two years ago Superman Returns director Bryan Singer was attached to a separate Warner Bros project based on the Randy Shilts 1982 biography, The Mayor of Castro Street: The Life and Times of Harvey Milk.
This year the studio's art-house arm, Warner Independent Pictures, brought in Singer's Usual Suspects writer Chris McQuarrie to work on a new draft of the script.
But McQuarrie is currently working on the Tom Cruise drama Valkyrie, making production less likely to happen before Singer starts his Superman sequel, Hollywood Reporter added.
Damon would play White, who shot Milk and mayor George Moscone in 1978.
After serving five years of a seven-year sentence, White committed suicide in 1985.